1. Field of the Invention
This invention relates to an infrared optical fiber which is used for transmitting the light produced by a high output infrared laser, such as a carbon dioxide laser.
2. Background of the Invention
The light which is produced by a carbon dioxide laser has a wavelength of 10.6 .mu.m which falls within the wavelengths of infrared radiation. Its wavelength is so long that it cannot be transmitted by a quartz glass fiber or any other type of fiber that is used for transmitting visible light or near infrared radiation.
There is hardly available any material that can effectively transmit light having such a wavelength.
Moreover, the light which is produced by a carbon dioxide laser is so powerful that if its is absorbed by an optical fiber during its transmission, it easily generates heat which damages the fiber.
A crystalline fiber formed from a metal halide (thallium, alkali or silver halide) and a glass fiber formed from chalcogenide glass are known as infrared fibers which can transmit the light produced by a high output carbon dioxide laser. These materials can transmit the light of a carbon dioxide laser with a relatively small loss. This loss is however, by far greater than the loss of visible light which occurs when it is transmitted through a quartz glass fiber.
An infrared fiber including a core formed from those materials is known which has a length of one to several meters.
While these materials are appropriate for the core of an infrared fiber, it has been very difficult to obtain an appropriate cladding material.
A quartz glass optical fiber, a multi-component glass optical fiber, and the like which are used for transmitting visible light or near infrared radiation are optical fibers of a double construction composed of a core and a cladding. The cladding is formed from a material which can transmit light and has a refractive index which is slightly lower than that of the core. The difference is refractive index between the core and the cladding determines the number of modes in which light is propagated through the core. While the cladding is required to have a refractive index which is slightly lower than that of the core, it is also required to be capable of transmitting light without absorbing it. A quartz glass fiber having such a difference in refractive index can be produced if an oxide is added to either its core or its cladding which are both formed from quartz.
A typical optical fiber has a core and a cladding. The cladding has the advantage of being capable of confining light effectively and ensuring that its propagation is free from the influence of any external factor. It is desirable to construct infrared fiber as a double structure which is composed of a core and a cladding. There has, however, not been available any infrared fiber having an appropriate cladding.
There are two reasons why no appropriate cladding has been available. In the first place, it is necessary to form a cladding from a material which has a slightly lower refractive index than that of the core and can transmit infrared radiation satisfactorily. However, no such material has hitherto been available. In the second place, it is difficult to coat the core with a cladding material.
Therefore, a crystalline infrared fiber has only a core and does not have any particular cladding. The surrounding air is its cladding. It is a fiber of the air clad construction which does not have any `tangible` cladding. However, air can be considered as a good cladding, since it can transmit infrared radiation satisfactorily and has a refractive index which is lower than that of the core.
There is also known a resin clad chalcogenide glass fiber having a teflon cladding. This teflon cladding is, however, different from an ordinary cladding, since it does not transmit any light produced by a carbon dioxide laser.
The air clad crystalline optical fiber and the resin clad chalcogenide glass optical fiber have presented a number of problems in connection with the transmission of light from a high output carbon dioxide laser, as will hereinafter be described.
If the air clad fiber contacts an object supporting it, it fails to transmit that amount of power which it can transmit when it does not contact the supporting object. This results in a drastic reduction of the power which it can transmit since light leaks out of the fiber where it contacts the supporting object.
Any attempt to transmit a higher power of light results in the instantaneous melt down of the fiber where the fiber contacts the supporting object. As no air exists where the fiber contacts the supporting object, light is not effectively confined but leaks out. The leaking light intensely heats the supporting object. The fiber is heated so intensely that it melts.
The air clad fiber is thus unreliable insofar as it employs air as its cladding. No such problem would occur if the fiber were out of contact with any other object. The fiber must, however, be supported somewhere. The problem of heat generation by the supporting objects is, therefore, difficult to avoid wherever the fiber may be supported.
The resin clad optical fiber employing teflon, etc. for its cladding is quickly heated during its transmission of light because its cladding absorbs light. Therefore, it is very likely to melt down.
The causes of heat generation and melt down will now be discussed. An infrared fiber for a carbon dioxide laser is always exposed to the danger of melt down as the laser produces a powerful beam of light. Discussion will, therefore, be made of the possible causes of such heat generation and melt down. A study of the distribution of an electromagnetic field in the modes in which light is propagated through an infrared fiber is believed to provide an understanding of the causes.
One of the modes in which light propagates through an optical fiber is known as the wave guide mode. According to this mode, light is propagated as a result of its total reflection on a boundary surface between the core and cladding of the fiber, as shown in FIG. 8. The maximum angle .theta. of reflection depends on the reflective indexes of the core and the cladding.
When light is totally reflected, it is true from a standpoint of geometrical optics that no light exists in the cladding. As a matter of fact, however, an electromagnetic field also extends into the cladding, as shown in FIG. 9. FIG. 9 shows the strength of the electric field in the core and the cladding.
It is also possible that rays of light may scatter for some rason or other during its propagation through the core, as shown in FIG. 10. The scattering of light results in a change of the angle of propagation and some rays having greater angles are propagated into the cladding without being reflected on the boundary surface between the core and the cladding. FIG. 11 shows the distribution of electric field strength resulting from such scattering of light. This mode, in which light is propagated into the cladding, is known as the radiation mode. The radiation mode is always likely to occur, as some factor or other that causes the scattering of light exists in any optical fiber and also because the fiber is sometimes bent.
Thus, the electromagnetic field has a portion extending into the cladding even when light is propagated in accordance with the wave guide mode, and according to the radiation mode, some electromagnetic radiation is propagated into the cladding without being totally reflected.
The electromagnetic radiation which is propagated into the cladding area of an air clad optical fiber is absorbed by an object contacting its core to support it and having a high power of absorbing light. As a result, intense heat is generated where the core contacts the supporting object.
The electromagnetic radiation which is propagated into the cladding of a teflon clad optical fiber is also absorbed by the cladding, as teflon has a high light absorbtion. The resulting heat energy intensely heats the cladding and the core.